We are studying the specific combining sites of antibody molecules and of certain myeloma proteins. Homogeneous populations of induced anti-hapten antibodies from individual animals are being identified in an extension of our previously obtained knowledge of the limited heterogeneity of such antibodies. Procedures are being used to isolate homogeneous antibodies by fractionating populations of limited heterogeneity or by stimulating the production of relatively homogeneous populations. The amino acid residues composing the binding sites are being determined by chemical alteration and affinity labeling techniques. The relationship of these residues to the subunit structure of immunoglobulins and to amino acid sequences in limited parts of the variable regions of the light and heavy chains will be determined. We are studying different antibodies directed against the same haptenic group as well as antibodies against different haptenic groups. The principal objective of the proposed research is to provide information on the relationship between the amino acid composition of the combining sites and the microspecificity of the sites in terms of their ability to bind homologous and cross-reactive haptenic structures. The studies are designed to lead to a determination of the tertiary structure of the various combining sites and to provide information on the potential of an individual to produce antibodies against a given determinant, to answer the questions of how many such antibody molecular species can be induced and what constitutes the difference, if any, in the fine specificity of the molecules.